Partially Crystalized Polyester Containers

ABSTRACT

A clear polyester container ( 1.0 ) comprising a base and upwardly extending side walls terminating in a continuous flange projecting from the side walls is described. Said multilayer container comprises an inner layer ( 1.1 ) in amorphous phase and an outer layer comprising a nucleating agent in the polymer composition in order to be crystallized in at least one portion of the flange.

The present invention relates to packages for products, preferably food products, comprising in particular clear polyester containers having a crystallized portion of the flange with good sealability to a top web.

The resultant package has a neat, clear appearance which is more attractive to the final consumer.

Commercially sold packages usually have a product support member, i.e. a container or a tray, with a cavity formed therein to accomodate the product and a flange around the perimeter of the member to be lidded, and the tray and the lid are not made by the same material causing recycling problems.

A generally known process to form a polyester tray includes extruding a polyester sheet in amorphous phase and subsequently crystallizing the whole tray by using a two stage mold, one hot for crystallization and the other cold for cooling down the article.

This kind of process results suitable to make mono-layer PET trays able to withstand the shrink of a lid without giving container distortion but the same process of crystallization makes the tray opaque and significantly reduces the rate of production due to thermoforming cycles.

STATE OF THE ART

U.S. Pat. No. 4,374,800 relates to a PAT (Polyalkylene terephthalate) container including a body portion of at least 20% of crystallinity, a flange having a crystallinity lower than that of the body, adjoined to the body portion.

EP 0 605 081 relates to a food package, comprising a crystalline opaque or colored tray thermoformed from a nucleated PET sheet which remains rigid at elevated temperature, and a transparent cover in an amorphous state connected to the tray.

GB 2 205 063 relates to a PET container having different degrees of crystallization in different regions of the article to ensure best functionality, the different degree of crystallization of the tray is obtained using a special mold at different heating region without using nucleating agents.

US 2007/0082151 relates to a polyester container with a flange with a projecting portion, which exhibits low-temperature heat sealability.

There is still the need of a clear polyester container with a flange showing good sealability and with a sufficient mechanical resistance to withstand the shrink force exerted by a shrinking lid.

We have now found that a clear polyester container with a good sealability on the flange and good mechanical properties can be obtained by crystallizing only a portion of the flange. The resultant container remains transparent while increasing the rigidity of the flange which can withstand the shrink of the lid and shows good sealability.

DESCRIPTION OF THE INVENTION

Therefore object of the present invention is a clear multilayer polyester container comprising a base and upwardly extending side walls terminating in a continuous flange projecting from the side walls, said multilayer container comprising at least an inner layer of an amorphous polyester and a bulk outer layer comprising a polyester in admixture with a nucleating agent, said bulk outer layer being crystallized in at least one portion of said flange.

For the purpose of the present invention the term “inner layer” means the layer of the container that will be in contact with the content of the container or in alternative that part of the container that comes in contact with the forming plug of the mold; in FIG. 1 the “inner layer” is indicated by (1.1), the crystallized portion of the flange and the bulk outer layer are indicated by (1.2) and (1.3), respectively.

For the purpose of the present description and of the claims which follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein

A container according to the present invention wherein said bulk outer layer is continuously crystallized all along the flange is particularly preferred.

In a preferred embodiment of the present invention the container is a double-layer or tri-layer container. In a double-layer container, the inner layer is in the amorphous phase and the crystallized portion of the flange is in the other outer layer. In a tri-layer container, the inner layer is in the amorphous phase and the crystallized portion of the flange is preferably in the intermediate bulk layer. In a tri-layer container according to the present invention the third outermost layer is preferably amorphous, being the nucleating agent preferably in the second layer. In the preferred embodiment only the portion of the bulk outer layer of the flange is crystallized while the other parts of the same, i.e. of the side walls and of the bottom part of the container, remain in amorphous phase, i.e. with a crystallinity percentage lower than 10%.

The polymers used in the present invention are clear amorphous thermoplastic polymers that can be injection molded or sheet extruded.

In particular the polymers used in the present application are polyesters, such as, for example polyethylene terephthalate (PET), polylactic acid (PLA), polyethylene terephthalate—glycol (PETG).

In a preferred embodiment the polymer used in the present invention is PET or PETG.

The inner layer remains amorphous during the crystallization of the flange so it can easily seal to a top lid using conventional sealing system, such as sealing bars. The bulk outer layer(s) comprising the nucleating agent can be crystallized in the flange part of the container, for example by using the mold showed in FIG. 2.

The nucleating agent used to promote crystallization is contained in the bulk outer layer of the container and preferably only the flange portion is crystallized.

Examples of nucleating agents which can be used in the container according to the present application and nucleating agent known in the art and commercially available, are:

-   -   (i) alkali metal salts of organic acids, e.g. carboxylic acid;         sodium, lithium, potassium benzoates (see D. Garcia,         Heterogeneous nucleation of PET, J. of Polymer Science—Polymer         physics edition, Vol 22, 2063-2072, 1984 and R. Legras, C.         Bailly, M. Daumerie, V. Zichy and others, Chemical nucleation, a         new concept applied to mechanism of action of organic acid salts         on the crystallization of PET and bisphenol-A-polycarbonate,         Polymer, Vol 25, 835-844, 1984) or sodium salts of substituted         benzoic acids which contain at least one nitro, halogen,         hydroxyl, phenyl or oxyphenyl substituent, and salts of alkali         metals including phenolic, phosphonic, phosphinic and sulfonic         (see EP 0 021 648);     -   (ii) lithium and/or sodium salts of aliphatic, cycloaliphatic,         aromatic carboxylic acids or heterocyclic polycarboxilic acids,         containing up to 20 carbon atoms (see U.S. Pat. No. 3,761,450);     -   (iii) DBS-dibenzylidene sorbitol (see J. of Applied Polymer         Science, Vol 36, 387-402, 1988);     -   (iv) trygliceride oil and triglyceride oil in combination         with/or chemically bonding to organic acid metal salts (see U.S.         Pat. No. 5,356,972)

The nucleating agent preferably used in the present invention is commercially available as Tna S471 by Sukano®.

The amount of nucleating agent generally depends on the type of polyester used. According to the conventional practice, the nucleating agent is generally used in an amount of from 2% to 5%, more preferably from 2.5% to 4%, even more preferably of 3%.

Preferably the nucleating agents have a particle size not higher than 10 microns.

The containers object of the present invention can be manufactured, for example, according to a process comprising the following steps:

-   -   a) providing a multilayer polyester sheet comprising an inner         amorphous polyester layer and a bulk outer layer comprising a         polyester in admixture with a nucleating agent;     -   b) pre-heating said multilayer polyester sheet at a temperature         between 90° and 120 ° C.     -   c) forming said preheated polyester sheet in a mold suitable for         inducing crystallization of the bulk outer layer in at least one         portion of the flange.

The crystallization of the flange is therefore selectively guaranteed by the heating provided on the flange using, for instance, a mold similar to the mold showed in FIG. 2.

The heating can be provided using a mold at different heating zones in order to crystallize only a portion of the flange or preferably the entire flange. The percentage of crystallization in the bulk outer layer outside the flange remains lower than the percentage of crystallization of the flange or portions of the flange.

The crystallization of flange may take place, preferably, in a forming mold but also outside a mold.

The percentage of crystallization of the flange or portion of the flange is equal to or higher than about 10%, preferably higher than 12%, more preferably higher than 15%. The crystallization of the flange takes place by heating the bulk outer layer generally at a temperature of from 270° C. to 300° C., preferably at 275° C. for a time comprised between 5 seconds and 10 seconds, preferably between 6 seconds and 8 seconds. The heat transfer takes place by conduction, induction or convection systems. The flange, after heating, can be cooled if needed.

The flange has a thickness ranging from 100 μm to 1000 μm, preferably from 200 μm to 700 μm, more preferably from 300 μm to 600 μm.

Furthermore the container object of the present invention, having the nucleating agent contained into the whole bulk outer layer, can also be crystallized if needed, in other portions but preferably only the flange is crystallized, thus providing the optical and mechanical properties needed i.e. more rigidity of the flange, transparency of the container and good sealability to the top lid.

The container of the present invention with the crystallized flange withstands the force imparted by the shrinking of lidding films and resists to deformations . However the present container is also suitable for tray lidding applications with non shrinkable films.

The container according to the present invention can be loaded with the product and then closed with a suitable top web by using a lidding machine which is characterized by a sealing bar operating at adequate temperature to get the seal. The temperature of the sealing bar depends on the type of lidding film used and the sealing operation can be carried out both under clean and contaminated conditions.

The packages so obtained, further object of the present invention, have a particularly good appearance. Moreover said packages, having the container and the lid the same polymeric composition, are really advantageous in the step of recycling. Furthermore it is possible to manufacture them by using recycled polyester.

EXAMPLES

A three-layer container (sample 1) in which the flange has a total thickness of 520 μm is formed using a mold as showed in FIG. 2.

The inner layer is made of 100% PETG 6763 by Eastman Chemical and has a thickness of 100 μm.

The second layer is made of 97% PETG Ramapet 9921W by Indorama and 3% of nucleating agent Tna S471 by Sukano® and has a thickness of 320 μm.

The third layer is made of 97% PETG Ramapet 9921W by Indorama and 3% of nucleating agent Tna S471 by Sukano® and has a thickness of 100 μm.

Sample 1 is tested in order to evaluate the variability of crystallinity and is tested after flange conditioning at different temperatures for different times.

The areas of the flange were selected and submitted to the thermal analysis that showed 12% of crystallinity, by conditioning the sample at 275° C. for 6 seconds.

The crystallinity of the flange is higher than the crystallinity of the container which is 2.45%.

DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a two layers container (1.0) according to the present invention. The inner layer (1.1) is in the amorphous phase while the bulk outer layer has two portions at different degrees of crystallization: the area of the crystallized flange (1.2) and the remaining area of the polyester in the amorphous state (1.3).

FIG. 2 represents the forming process used to provide the container according to the present invention.

The polyester film (2.1) is inserted into the female mold which comprises the cooled part (2.3) and the heated part (2.2) where the crystallization of the flange occurs.

The polyester film (2.1) adheres to the female mold due to the vacuum applied (2.5), while the forming plug (2.4) pushes the polyester film (2.1) in the female mold, providing the final container. 

1) A clear multilayer polyester container comprising a base and upwardly extending side walls terminating in a continuous flange projecting from the side walls, said multilayer container comprising at least an inner layer of an amorphous polyester and a bulk outer layer comprising a polyester in admixture with a nucleating agent, said bulk outer layer being crystallized in at least one portion of said flange. 2) The container according to claim 1 wherein said bulk outer layer is continuously crystallized all along the flange. 3) The container according to claim 1 wherein the polyester is selected among polyethylene terephthalate, polylactic acid, and polyethylene terephthalate-glycol. 4) The container according to claim 3 wherein the polyester is polyethylene terephthalate or polyethylene terephthalate-glycol. 5) The container according to claim 1 having two or three layers of polyester film. 6) The container according to claim 1 wherein the flange has a thickness of from 100 μm to 1000 μm. 7) The container according to claim 1 wherein the percentage of crystallization of the crystallized portion of the flange is equal to or higher than about 10%. 8) The container according to claim 1 wherein the crystallization of the flange is obtained by heating at least one portion of said flange at a temperature of from 270° C. to 300° C. for a time from 5 seconds to 10 seconds. 9) The container according to claim 1 wherein the nucleating agent is selected among alkali metal salts of organic acids; lithium or sodium salts of aliphatic, cycloaliphatic, aromatic carboxylic acids or heterocyclic polycarboxylic acids, containing up to 20 carbon atoms; DBS-Dibenzylidene Sorbitol; trygliceride oil and triglyceride oil in combination with organic acid metal salts or chemically bonding to organic metal salts. 10) A package comprising a container according to claims 1, including a product loaded into said container and a lid sealed onto the flange of said container. 11) The package according to claim 10 wherein the container and the lid are made of polyesters. 12) The package according to claim 10 wherein said lid is made of a shrinkable film. 13) The package according to claim 10 having one or more films covering it. 14) Use of a container according to claim 1 for tray lidding applications. 15) A process for manufacturing a container, comprising the steps of: a) providing a multilayer polyester sheet comprising an inner amorphous polyester layer and a bulk outer layer comprising a polyester in admixture with a nucleating agent; b) pre-heating said multilayer polyester sheet at a temperature between 90 and 120 ° C.; and c) forming said pre-heated polyester sheet in a mold suitable for inducing crystallization of the bulk outer layer in at least one portion of the flange. 